Keep in mind that with a mesh setup the number of VPN tunnels need goes up really quickly. Make sure your MX's have a high enough number of supported VPN tunnels to handle the number needed. How many branches does your client have, and how many uplinks does each MX use?

Re: Meraki Support Down?

Okay 4-5 branches should be okay on an MX84 full mesh setup (even with 2 WAN-uplinks).

An exit hub means that all traffic that would normally go to the default route will in fact be sent to that exit hub instead. Effectively this concentrates all your traffic to a single break-out point. In your case I think this is exactly what you don't want. You want traffic destined for the internal subnets to be transferred over the VPN but other traffic destined for the internet to break out locally.